The present invention relates to a control system for an internal combustion engine, and more particularly to an air flow control apparatus for controlling an amount of air flow under control of a throttle valve into the engine.
In such a conventional air flow control apparatus as disclosed, for example, in a Japanese Patent Early Publication No.55-156230, an amount of the air flow in loaded condition of the engine is adjusted in open-loop control in dependence upon temperature of cooling water of a cooling system for the engine. When the engine is conditioned in idling operation, operation of the apparatus is switched from the open-loop control to feedback control such that an amount of the air flow is adjusted in accordance with the actual idling speed of the engine to bring the actual and desired idling speeds in accord. In this instance, an initial value of an amount of the air flow at the switchover timing from the open-loop control to the feedback control is determined based on an amount of the air flow in open-loop control, taking account of temperature of the cooling water and the loaded condition of the engine, to ensure smooth idling speed of the engine.
However, in such an air flow control apparatus, it is recognized that an amount of the air flow into the engine is generally adjusted based on density of air corresponding to a standard atmospheric pressure of 760 mmHg at a low place where an automotive vehicle travels. This means that an amount of the adjusted air flow at a high place, where the vehicle travels, will substantially decrease in comparison with that at the low place, because density of air decreases due to lowering of the atmospheric pressure caused by rise in height of the vehicle travel place. As a result, it is observed that when operation of the control apparatus is switched from the open-loop control to the feedback control after travelling of the vehicle under the open-loop control at a stretch from the low place to the high place, the above-mentioned substantial decrease in density of air induces undesired lowering of idling speed or stop of the engine, as understood from FIG. 1.